A popular process for the manufacture of two-phase charge coupled devices involves the implantation of arsenic selectively into a silicon oxide layer on the surface of a silicon substrate for subsequent outdiffusion from the oxide layer into the underlying silicon substrate.
Devices of this kind using diffusion out of the oxide to control the doping in the underlying silicon to control the channel potential of the silicon are found susceptible to several problems.
A first problem arises from charge trapping in the oxide due to residual arsenic that does not diffuse completely out of the oxide. Such charge trapping can cause threshold voltage shifts, drift in operating characteristics and low breakdown voltages in the finished device.
A second problem arises because the diffusion rate of the arsenic out of the oxide tends to vary so that the channel potential resulting from the diffusion is poorly reproducible.
A third problem is the occurrence of potential defects associated with arsenic diffusion out of the oxide under the first-phase set of gate electrodes. Such defects can reduce the efficiency of charge transfer and seriously degrade device performance.